1. Field of the Invention
The present invention relates generally to a liquid crystal display device. More particularly, the present invention relates to a double-layered type white/black super-twisted nematic liquid crystal display device including a heater.
2. Description of the Related Art
When a white/black super-twisted nematic liquid crystal display device having a double layered-structure is used at a lower temperature, it is required that the liquid display device is heated up to an elevated temperature.
To facilitate understanding of the present invention, a typical conventional (prior art) liquid crystal display device will briefly be described below with reference to FIG. 3.
In FIG. 3, reference numeral 111 designates a driving cell, reference numeral 112 designates a compensating cell, reference numerals 113 and 114 designate a polarization plate, respectively, and reference numeral 115 designates an illumination light source (back light). A heater 116 is attached to the polarization plate 113 on the driving cell 111 side.
The reason why the heater 116 is attached to the driving cell 111 side is described in the following.
An electric conductive film of an indium tin oxide, a gold or the like material deposited on the surface of an uniaxial polyester sheet by employing a vaporizing process or a spattering process is normally used to serve as a heater for the liquid crystal display device. If the heater 116 is attached to the compensating cell 112 side, the polarized state is undesirably disturbed and thereby optical characteristics of the liquid crystal display device such as a display contrast, a visual angle or the like are deteriorated remarkably.
Although the heater 116 is attached to the driving cell 111 side as shown in FIG. 3, it has been found that there arises a problem that a malfuncion of coloring occurs due to the presence of a differential temperature between the two cells.
Alternatively, arrangement of the driving cell 111 and the compensating cell 112 may be inverted relative to the illumination light source 115. In this case, the heater 116 is attached to the compensating cell 112 side, i.e., the light source 115 side. However, it has been found that there arises another problem that a property of response is degraded.
Additionally, when the heater is attached to the observer A side, an observer A directly views the heater, resulting in a quality of displaying being deteriorated remarkably.
Another conventional liquid crystal display device (not shown) is constructed such that a heater comprising such an electric conductive film as mentioned above is attached to each of the driving cell and the compensating cell.
In this case, a heating cost is increased twice. In addition, when temperature increase on the driving cell side differs from temperature increase on the compensating side due to fluctuation in electricity consumed by each of the heaters, there is a possibility that a quality of displaying is degraded. Further, since the liquid crystal display device is provided with two control systems each including a temperature sensor, circuits for the liquid crystal display device become unavoidably complicated in structure, causing the liquid crystal display device to be fabricated at an increased cost. Another problem is that an optical penetration rate is remarkably reduced because of arrangement of two heaters.